1. Field of the Invention
This invention relates to a device and method for separating heavier and lighter fractions of a fluid sample. More particularly, this invention relates to a device and method for collecting and transporting fluid samples whereby the device and fluid sample are subjected to centrifugation to cause separation of the heavier fraction from the lighter fraction of the fluid sample.
2. Description of Related Art
Diagnostic tests may require separation of a patient""s whole blood sample into components, such as serum or plasma, the lighter phase component, and red blood cells, the heavier phase component. Samples of whole blood are typically collected by venipuncture through a cannula or needle attached to a syringe or an evacuated collection tube. Separation of the blood into serum or plasma and red blood cells is then accomplished by rotation of the syringe or tube in a centrifuge. Such arrangements use a barrier for moving into an area adjacent the two phases of the sample being separated to maintain the components separated for subsequent examination of the individual components.
A variety of devices have been used in collection devices to divide the area between the heavier and lighter phases of a fluid sample.
The most widely used device includes thixotropic gel materials such as polyester gels in a tube. The present polyester gel serum separation tubes require special manufacturing equipment to prepare the gel and to fill the tubes. Moreover, the shelf-life of the product is limited in that over time globules may be released from the gel mass. These globules have a specific gravity that is less than the separated serum and may float in the serum and may clog the measuring instruments, such as the instrument probes used during the clinical examination of the sample collected in the tube. Such clogging can lead to considerable downtime for the instrument to remove the clog.
No commercially available gel is completely chemically inert to all analytes. If certain drugs are present in the blood sample when it is taken, there can be an adverse chemical interaction with the gel interface.
Therefore, a need exists for a separator device that (i) is easily used to separate a blood sample; (ii) is independent of temperature during storage and shipping; (iii) is stable to radiation sterilization; (iv) employs the benefits of a thixotropic gel barrier yet avoids the many disadvantages of placing a gel in contact with the separated blood components; (v) minimizes cross contamination of the heavier and lighter phases of the sample during centrifugation; (vi) minimizes adhesion of the lower and higher density materials against the separator device; (vii) can be used with standard sampling equipment; (viii) is able to move into position to form a barrier in less time than conventional methods and devices; and (ix) is able to provide a clearer specimen with less cell contamination than conventional methods and devices.
The present invention is a method and assembly for separating a fluid sample into a higher specific gravity phase and a lower specific gravity phase. Desirably, the assembly of the present invention comprises a plurality of constituents. Preferably, the assembly comprises a container and a composite element.
Most preferably, the container is a tube and the composite element is a separator arranged to move in the tube under the action of centrifugal force in order to separate the portions of a fluid sample. Preferably, the separator is a radically deformable seal plug.
Most preferably, the separator is used within a container such as a tube. The tube comprises an open end, a closed end and a sidewall extending between the open end and closed end. The sidewall comprises an outer surface and an inner surface. The tube further comprises a closure disposed to fit in the open end of the tube with a resealable septum. Preferably, the separator element is releaseably positioned at the open end of the tube with the bottom area of the closure.
Most preferably, the separator consists of two components. Preferably, the composite element comprises an overall specific gravity at a target specific gravity of "sgr"t. The target specific gravity is that required to separate a fluid sample into at least two phases. Preferably, the separator comprises at least two or more regions of differing specific gravities. Preferably, at least one of the regions is higher than the target specific gravity and at least one of the regions is lower than the target specific gravity.
Desirably, the separator comprises a low density plastic body and a high density elastomeric seal diaphragm. The separator as a whole is sized to fit releasably within the tube with an interference fit against the inner surface of the sidewall of the tube.
Preferably, the plastic body has a hollow core or a central passage and a hollow chamber surrounding the hollow core. The plastic body has a specific gravity of about 1.1 to about 7.9.
Desirably, the plastic body is a substantially rigid moldable thermoplastic material such as polystyrene, polyethylene, polypropylene, and mixtures thereof that is inert to the fluid sample of interest.
Desirably, the elastomeric seal diaphragm comprises a fill septum to facilitate needle penetration during the blood fill cycle and an annular skirt that is able to deform during the centrifugation process and reduce the overall diameter of the separator.
Desirably, the plastic body is nested within the elastomeric seal diaphragm, whereby the channel of the plastic body is in direct communication with the fill septum of the elastomeric seal diaphragm.
Desirably, the elastomeric seal diaphragm may be comprised of any natural or synthetic elastomer or mixture thereof, that is inert to the fluid sample of interest. Preferably, the elastomeric seal diaphragm is made from an elastomer having a 50% tensile modulus from about 0.1 MPa to about 1.4 MPa.
Preferably, the separator has a density of about 1.028 to about 1.09 g/cc so that the separator will come to rest after centrifugal force, at substantially at the border between the heavier and lighter phases of the fluid sample under consideration, and most preferably between the heavier and lighter phases of a blood sample. Preferably, the separator will function under load created by an applied acceleration of about 300 g to about 3000 g.
Preferably, the separator is initially secured to the bottom area of the closure. Desirably, the bottom area of the closure includes an integrally molded gripping means for releasably holding the elastomeric seal diaphragm of the separator with the closure until the assembly is subjected to centrifugation at which time the separator is released from the gripping means of the closure. The separator is further fitted with the tube whereby the annular skirt of the elastomeric seal diaphragm, which provides the largest diameter of the separator in its undeformed state, has an interference fit with the inner surface of the sidewall of the tube.
In use, a fluid sample enters the assembly by needle. The needle penetrates the closure and the elastomeric seal diaphragm of the separator. The sample enters the assembly through the needle and through the channel of the plastic body and into the body of the tube. The needle is withdrawn from the assembly and the septum of the closure and the elastomeric seal diaphragm reseal.
The assembly is then subjected to centrifugation. Under centrifugation, the separator is released from the closure. The separator migrates axially down the tube towards the closed end. The migration is facilitated by the density of the plastic body versus the density of the elastomeric seal diaphragm of the separator. The annular skirt of the elastomeric seal diaphragm temporarily deforms under centrifugation, whereby the diameter is reduced eliminating its interference fit with the inner wall surface of the tube. Therefore, a path is developed between the inner wall of the tube and the separator that permits the flow of the low density component past the separator as it migrates down the tube. Migration of the separator terminates when it reaches the position between the lower density liquid component and higher density cellular/solid components, equal to its overall density. Upon terminating centrifugation, the annular skirt of the elastomeric seal diaphragm returns to its undeformed shape, sealing against the inner wall of the tube, thereby creating a barrier between the higher and lower density components of the fluid.
The separator""s position at the top of the tube in conjunction with the closure and the elastomeric seal diaphragm""s penetrable area into the passage throughway of the separator, provides easy direct loading of the fluid sample into the tube without any obstructions. Thus, the fluid sample is easily delivered into the tube without exposing the uncentrifuged fluid sample to the outer surface area of the separator.
However, it is within the purview of the invention that the separator may be placed at the bottom of the tube as well.
Thus, when the container is subjected to centrifugation, the separator moves away from the grip of the closure and towards a position between the lower and higher specific gravity phases of the fluid sample.
When the fluid sample is blood, the higher specific gravity portion that contains the cellular components is between the separator and the bottom of the container after centrifugation. The lower specific gravity portion that contains the cell free serum fraction or plasma is between the top surface of the separator and the top of the container after centrifugation.
Therefore, at the final position of the separator after centrifugation, the separator is able to substantially eliminate the presence of red blood cells in the lower specific gravity portion and the lower specific gravity portion is substantially free of cellular contamination.
The assembly of the present invention is advantageous over existing separation products that use gel. In particular the assembly of the present invention will not interfere with analytes as compared to gels that may interfere with certain analytes. Another attribute of the present invention is that the assembly of the present invention will not interfere with therapeutic drug monitoring analytes.
Most notably, the time to separate a fluid sample into separate densities is achieved in substantially less time with the assembly of the present invention as compared to assemblies that use gel.
Another notable advantage of the present invention is that fluid specimens are not subjected to low density gel residuals that are at times available in products that use gel.
A further attribute of the present invention is that there is no interference with instrument probes.
Another attribute of the present invention is that samples for blood banking tests are more acceptable than when a gel separator is used.
Another attribute of the present invention is that only the substantially cell-free serum fraction of a blood sample is exposed to the top surface of the separator, thus providing practitioners with a clean sample.
Additionally, the assembly of the present invention does not require any additional steps or treatment by a medical practitioner, whereby a blood or fluid sample is drawn in the standard fashion, using standard sampling equipment.